The present invention relates to industrial control systems used for real-time control of industrial processes, and in particular to an input/output (I/O) module for connection to thermocouples to provide for temperature measurement.
Industrial control systems are special purpose computer systems used in controlling industrial processes. Under the direction of a stored control program, a programmable logic controller, being part of the industrial control system, reads inputs from one or more I/O modules and writes outputs one or more I/O modules. The inputs are derived from signals obtained from sensors associated with the industrial process and the output signals produce electrical signals to actuators and the like in the industrial process. The inputs and outputs may be binary, that is on or off or analog, providing a value with a continuous range for more complex I/O devices like motor controllers and the like.
One form of analog input I/O module receives an input from a thermocouple. As is understood in the art, thermocouples provide a voltage that is proportional to a difference in temperature between two junctions of dissimilar metals per the Seebeck effect. In order to determine a temperature at one junction (“hot junction”), the second junction (“cold junction”) may be held at a standard and known temperature. For practical devices, however, this cold junction is not held at a particular temperature but rather its temperature is measured and used to provide for “cold junction compensation” in which to measure temperatures applied to empirically derived compensation tables that may be used to correct the value of the hot junction. These tables may also be used to correct for inherent nonlinearities in the voltage-to-temperature function of the thermocouple.
It is known to provide I/O modules for use with thermocouples with a cold junction compensation circuit providing a precision temperature measuring element, for example, a resistive thermal device (RTD) that may be attached to terminals near the terminals receiving signals from thermocouples to provide a measurement of the temperature in the vicinity of those terminals. This temperature measurement is used for the cold junction compensation by the I/O module through computations typically performed by a processor internal to the I/O module.
The cost of these precision temperature references is relatively high and they take up terminal space that could be used for other devices and accordingly a typical I/O module will use one or two cold junction compensation circuits at most.